Method and apparatus for friction stir welding

ABSTRACT

The present invention provides a tool for forming a friction stir weld joint in a workpiece. According to one embodiment, the tool includes a rotatable pin having first and second ends and defining a stirring portion therebetween structured to frictionally engage the workpiece so as to at least partially form the friction stir weld joint. The tool includes a rotatable first shoulder defining an aperture therethrough structured to slidably receive the first end of the pin. The tool also includes a second shoulder defining an aperture structured to receive the second end of the pin such that the pin extends between the first and second shoulders and such that the second shoulder is in rotatable communication with the pin. The first shoulder is structured to rotate independently of the pin and the second shoulder.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/237,506, filed Sep. 9, 2002, now U.S. Pat. No. 6,908,690 whichclaimed the benefit of U.S. Provisional Application No. 60/376,758,filed Apr. 29, 2002, which is hereby incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to friction welding and, morespecifically, to backing up a weld joint during friction stir welding.

2. Description of Related Art

Friction stir welding is a relatively new process using a rotating tool,which includes a threaded pin or probe attached to a concave shoulder,to join in a solid state two workpieces or to repair cracks in a singleworkpiece. At present, the process is applied almost exclusively instraight-line welds. For example, such a process is described in U.S.Pat. No. 5,460,317 to Thomas et al., the contents of which areincorporated herein by reference. As shown in FIG. 1A, during frictionstir welding, the probe 10 of the rotating tool 12 is plunged into aworkpiece or between two workpieces 14 by a friction stir weldingmachine (not shown) to produce the required resistance force to generatesufficient frictional heating to form a region of plasticized material.As shown in FIG. 1B, the tool 12 is typically tilted approximately 3°relative to the workpiece or workpieces 14 such that the trailing edgeof the concave shoulder 16 is thrust into and consolidates theplasticized material. Upon solidification of the plasticized material,the workpieces 14 are joined along the weld joint 18. The magnitude offorce exerted by the friction stir welding tool 12 must be maintainedabove a prescribed minimum in order to generate the required frictionalheating.

To prevent deformation of a workpiece by the force exerted by thefriction stir welding tool 12 and maintain dimensional tolerances, theworkpiece 14 must have support 15 behind the weld joint. Additionally,because the frictional heat generated by the welding tool plasticizesthe material within the weld joint, the plasticized material must beconstrained to prevent the material from flowing out of the weld jointand also must be consolidated to minimize porosity and provide a weldjoint having the desired surface finish. When friction stir weldingrelatively flat workpieces, the weld joint can be supported by acontinuous planar surface, such as a steel plate, positioned underneaththe workpieces to be joined.

When friction stir welding large workpieces or workpieces havingcurvilinear geometries, providing adequate support to the weld jointbecomes problematic because the amount of support material necessaryand/or the curvilinear geometry makes it more difficult and expensive toprovide a continuous support surface. Such welds are often necessarywhen fabricating military and commercial aircraft and rocket fuel tanks.In certain instances, a built-up structure, commonly referred to as“tooling,” can be secured to the interior surfaces of the workpiecesprior to friction stir welding. However, weight restrictions and/ordesign parameters often require a finished assembly having a smoothinterior surface. As such, the tooling must be removed, for example, bymachining, which is time consuming and labor intensive and increases themanufacturing cost of the finished assembly.

Another problem that has been encountered with friction stir weldingoccurs when joining workpieces formed of different materials havingdifferent material properties, such as solidus temperature, hardness,and/or thermal conductivity. The “solidus” temperature of a particularalloy is the temperature below which only a solid is stable. Thedifferent material properties can require that the friction stir weldingtool be rotated within each workpiece at different rotational speedsand/or have different rates of tool advance through the workpieces,which can complicate the friction stir welding process and can limit thetypes of materials that can be joined. For example, when friction stirwelding workpieces with different solidus temperatures, the frictionstir welding tool will plasticize the workpiece with the lower solidustemperature first, such that the workpiece with the higher solidustemperature may not be sufficiently plasticized to be mixed with theother workpiece, as is necessary to form a strong weld joint.

Thus, there is a need for an improved friction stir welding tool forforming weld joints between large workpieces or workpieces havingcurvilinear geometries. The tool should be capable of effectivelysupporting a weld joint and constraining the plasticized material withinthe weld joint during friction stir welding and should be easilyadaptable to varying workpiece geometries and sizes. In addition, thetool should allow for friction stir welding workpieces having differentmaterial properties.

SUMMARY OF THE INVENTION

The present invention provides a tool for forming a friction stir weldjoint in a workpiece. According to one embodiment of the presentinvention, the tool includes a rotatable pin having first and secondends and defining a stirring portion therebetween structured tofrictionally engage the workpiece so as to at least partially form thefriction stir weld joint. The tool includes a rotatable first shoulderdefining an aperture therethrough structured to slidably receive thefirst end of the pin. The tool also includes a second shoulder definingan aperture structured to receive the second end of the pin such thatthe pin extends between the first and second shoulders and such that thesecond shoulder is in rotatable communication with the pin. The firstshoulder is structured to rotate independently of the pin and the secondshoulder. In one embodiment, the first and second shoulders arestructured to opposingly support the workpiece during friction stirwelding. In another embodiment, the second shoulder defines a pluralityof fins adapted to transfer heat away from the weld joint.

According to another embodiment of the present invention, the toolincludes a rotatable pin having first and second ends and defining astirring portion therebetween structured to frictionally engage theworkpiece so as to at least partially form the friction stir weld joint.The tool includes a rotatable first shoulder defining an aperturetherethrough structured to slidably receive the first end of the pin.The tool also includes a second shoulder defining an aperture structuredto receive the second end of the pin such that the pin extends betweenthe first and second shoulders and such that the second shoulder is inrotatable communication with the pin. At least one of the first andsecond shoulders has a surface defining at least one raised portionstructured to frictionally engage the workpiece so as to at leastpartially form the friction stir weld joint. The first and secondshoulders are structured to opposingly support the workpiece duringfriction stir welding. In one embodiment, the first shoulder isstructured to rotate independently of the pin and the second shoulder.In another embodiment, the second shoulder defines a plurality of finsadapted to transfer heat away from the weld joint.

According to another embodiment of the present invention, the toolincludes a rotatable pin having first and second ends and defining astirring portion therebetween structured to frictionally engage theworkpiece so as to at least partially form the friction stir weld joint.The tool includes a rotatable first shoulder defining an aperturetherethrough structured to slidably receive the first end of the pin.The tool also includes a second shoulder defining an aperture structuredto receive the second end of the pin such that the pin extends betweenthe first and second shoulders and such that the second shoulder is inrotatable communication with the pin. The second shoulder defines aplurality of fins adapted to transfer heat away from the weld joint. Inone embodiment, the first and second shoulders are structured toopposingly support the workpiece during friction stir welding. Inanother embodiment, the first shoulder is structured to rotateindependently of the pin and the second shoulder.

The stirring portion of the pin can comprise a variety ofconfigurations. For example, in one embodiment, the stirring portion ofthe pin defines at least one planar surface. In another embodiment, thestirring portion of the pin defines at least one threaded surface and atleast one planar surface. In still another embodiment, the stirringportion of the pin comprises a first threaded surface having threadsoriented in a first direction and a second threaded surface havingthreads oriented in a second direction, and wherein the first directionis different from the second direction.

The first and second shoulders each has a surface structured tofrictionally engage the workpiece to thereby at least partially form thefriction stir weld joint. For example, in one embodiment, at least oneof the surfaces of the first and second shoulders is threaded. Inanother embodiment, at least one of the surfaces is convex. In stillanother embodiment, at least one of the surfaces is concave. In yetanother embodiment, at least one of the surfaces defines at least oneraised portion structured to frictionally engage the workpiece.

The pin can be connected to the second shoulder in a variety ofdifferent ways. For example, in one embodiment, at least a portion ofthe second end of the pin is threaded. Similarly, at least a portion ofthe aperture of the second shoulder is threaded so as to threadablyreceive the second end of the pin. In another embodiment, the second endof the pin has a polygonal configuration and the aperture of the secondshoulder has a polygonal configuration corresponding to theconfiguration of the second end of the pin.

The present invention also provides an apparatus for forming a frictionstir weld joint in a workpiece. According to one embodiment, theapparatus includes a machine having a spindle defining a rotatable innerportion and a rotatable outer portion. The apparatus includes a frictionstir welding tool. In one embodiment, the friction stir welding toolincludes a pin having first and second ends and defining a stirringportion therebetween structured to frictionally engage the workpiece soas to at least partially form the friction stir weld joint. The firstend of the pin is in rotatable communication with the inner portion ofthe spindle. The friction stir welding tool includes a first shoulderdefining an aperture therethrough structured to slidably receive thefirst end of the pin. The first shoulder is in rotatable communicationwith the outer portion of the spindle. The friction stir welding toolalso includes a second shoulder defining an aperture structured toreceive the second end of the pin such that the pin extends between thefirst and second shoulders and such that the second shoulder is inrotatable communication with the pin. The first shoulder is structuredto rotate independently of the pin and the second shoulder. As describedabove, many variations and modifications of the friction stir weldingtool and first and second shoulders are possible.

The present invention also provides a method of friction stir welding aworkpiece. According to one embodiment of the present invention, themethod comprises positioning first and second shoulders adjacent theworkpiece. Each of the first and second shoulders has a surfacestructured to frictionally engage the workpiece. A pin is connected tothe first and second shoulders so that the pin extends therebetween. Thepin defines a stirring portion structured to frictionally engage theworkpiece. Thereafter, the first shoulder is rotated at a first angularvelocity and the pin and the second shoulder are rotated at a secondangular velocity different from the first angular velocity so that atleast a portion of each of the pin, first shoulder, and second shoulderfrictionally engages the workpiece to thereby form a friction stir weldjoint. The stirring portion of the pin can be moved through theworkpiece along a predetermined path.

According to another embodiment of the present invention, the methodincludes positioning first and second shoulders adjacent the workpiece.Each of the first and second shoulders has a surface structured tofrictionally engage the workpiece. A pin is connected to the first andsecond shoulders so that the pin extends therebetween. The pin defines astirring portion structured to frictionally engage the workpiece. Thefirst shoulder is rotated. Concurrently with the first rotating step,the pin and the second shoulder are rotated independently of the firstshoulder so that at least a portion of each of the pin, first shoulder,and second shoulder frictionally engages the workpiece to thereby form afriction stir weld joint. For example, the first and second rotatingsteps can include rotating the first shoulder at a first angularvelocity and the pin and the second shoulder at a second angularvelocity, wherein the second angular velocity is different from thefirst angular velocity. As used herein, “angular velocity” includes botha speed component and a direction component. The direction component ispositive for motion following the “right hand rule,” i.e.,counter-clockwise motion, and is negative for motion in the oppositedirection, i.e., clockwise motion. The stirring portion of the pin canbe moved through the workpiece along a predetermined path.

The method of connecting the pin to the first and second shoulders canbe varied. In one embodiment, the connecting step comprises sliding anend of the pin through an aperture in the first shoulder. The connectingstep can then include threading an end of the pin into a threadedaperture defined by the second shoulder. In another embodiment, theconnecting step comprises drilling an aperture in the workpiece. An endof the pin is slid through an aperture in the first shoulder. The end ofthe pin is then inserted through the aperture in the workpiece andconnected to the second shoulder.

The position of the first and second shoulders relative to one anothercan be modified in order to adjust the force exerted by the shoulders onthe workpiece. For example, in one embodiment, the pin is urged towardthe first shoulder so as to urge the second shoulder toward the firstshoulder. In another embodiment, the first shoulder is urged toward thesecond shoulder.

The present invention also provides a friction stir weld lap joint beingformed by a rotating friction stir welding tool. The lap joint includesa first structural member and a second structural member. The secondstructural member at least partially overlaps the first structuralmember so as to define an interface therebetween. The lap joint includesa friction stir weld joint joining the first and second structuralmembers at least partially at the interface. The friction stir weldjoint defines first and second portions. The first portion of thefriction stir weld joint is mixed by the friction stir weld tool at afirst angular velocity and the second portion of the friction stir weldjoint is mixed by the friction stir welding tool at a second angularvelocity to thereby form grain structures of different refinement in thefirst and second portions. In one embodiment, the first and secondstructural members comprise different materials. In another embodiment,the first and second structural members have different solidustemperatures. In yet another embodiment, the first and second structuralmembers have different hardness.

Thus, there has been provided a friction stir welding tool, apparatusand associated method of manufacture for forming weld joints by frictionstir welding large workpieces or workpieces having curvilineargeometries. The tool is capable of effectively supporting a weld jointand constraining the plasticized material within the weld joint duringfriction stir welding. The tool can easily be adapted to varyingworkpiece geometries and sizes. In addition, the tool allows forfriction stir welding workpieces having different material properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention, andthe manner in which the same are accomplished, will become more readilyapparent upon consideration of the following detailed description of theinvention taken in conjunction with the accompanying drawings, whichillustrate preferred and exemplary embodiments, but which are notnecessarily drawn to scale, wherein:

FIG. 1A is a partial perspective view of a conventional friction stirwelding tool, illustrating the positioning of the shoulder and probeduring friction stir welding;

FIG. 1B is a cross-sectional view of a conventional friction stirwelding tool, illustrating the positioning of the shoulder and probeduring friction stir welding;

FIG. 2A is a perspective view illustrating the positioning of twoworkpieces to form an interface therebetween prior to friction stirwelding, according to one embodiment of the present invention;

FIG. 2B is a perspective view illustrating a structural assembly beingformed by friction stir welding the two workpieces shown in FIG. 2A,according to one embodiment of the present invention;

FIG. 3 is an elevation view illustrating a friction stir welding tool,according to one embodiment of the present invention;

FIG. 4 is an elevation view illustrating a pin for a friction stirwelding tool, according to one embodiment of the present invention;

FIG. 5 is an elevation view illustrating one end of the pin shown inFIG. 4;

FIG. 6 is an elevation view illustrating a pin for a friction stirwelding tool, according to another embodiment of the present invention;

FIG. 7 is a sectional view illustrating the pin shown in FIG. 6 alongline 7-7;

FIG. 8 is an elevation view illustrating a pin for a friction stirwelding tool, according to another embodiment of the present invention;

FIG. 9 is a sectional view illustrating the pin shown in FIG. 8 alongline 9-9;

FIG. 10 is an elevation view illustrating a pin for a friction stirwelding tool, according to another embodiment of the present invention;

FIG. 11 is a sectional view illustrating the pin shown in FIG. 10 alongline 11-11;

FIG. 12 is an elevation view illustrating a pin for a friction stirwelding tool, according to another embodiment of the present invention;

FIG. 13 is an elevation view illustrating one end of the pin shown inFIG. 12;

FIG. 14 is a sectional view illustrating the pin shown in FIG. 12 alongline 14-14;

FIG. 15 is an elevation view illustrating a pin for a friction stirwelding tool, according to another embodiment of the present invention;

FIG. 16 is an elevation view illustrating one end of the pin shown inFIG. 15;

FIG. 17 is a sectional view illustrating the pin shown in FIG. 15 alongline 17-17;

FIG. 18 is an elevation view illustrating the second shoulder of afriction stir welding tool, according to one embodiment of the presentinvention;

FIG. 19 is a plan view illustrating the end of the second shoulder shownin FIG. 18 structured for frictionally engaging a workpiece;

FIG. 20 is a bottom view illustrating the second shoulder shown in FIG.18;

FIG. 21 is a sectional view illustrating the second shoulder shown inFIG. 18 along line 21-21;

FIG. 22 is a partial elevation view illustrating the end of a pin of thefriction stir welding tool structured for connecting to the secondshoulder, according to one embodiment of the present invention;

FIG. 23 is an elevation view illustrating the second shoulder of afriction stir welding tool, according to another embodiment of thepresent invention;

FIG. 24 is a plan view illustrating the end of the second shoulder shownin FIG. 23 structured for frictionally engaging a workpiece;

FIG. 25 is a bottom view illustrating the second shoulder shown in FIG.23;

FIG. 26 is a sectional view illustrating the second shoulder shown inFIG. 23 along line 26-26;

FIG. 27 is an elevation view illustrating the end of the first shoulderof the friction stir welding tool defining an aperture for slidablyreceiving the pin, according to one embodiment of the present invention;

FIG. 28 is a sectional view illustrating the first shoulder shown inFIG. 27 along line 28-28;

FIG. 29 is an elevation view illustrating the end of the first shouldershown in FIG. 28 along line 29-29 structured for frictionally engaging aworkpiece;

FIG. 30 is a partial sectional view illustrating the end of the firstshoulder shown in FIG. 29 along line 30-30;

FIG. 31 is an elevation view illustrating the end of the first shoulderof the friction stir welding tool defining an aperture for slidablyreceiving the pin, according to another embodiment of the presentinvention;

FIG. 32 is a sectional view illustrating the first shoulder shown inFIG. 31 along line 32-32;

FIG. 33 is an elevation view illustrating the end of the first shouldershown in FIG. 32 along line 33-33 structured for frictionally engaging aworkpiece;

FIG. 34 is a partial sectional view illustrating the end of the firstshoulder shown in FIG. 33 along line 34-34;

FIG. 35 is an elevation view of a friction stir weld lap joint,according to one embodiment of the present invention;

FIG. 36 is a flow chart illustrating the operations for friction stirwelding, according to one embodiment of the present invention; and

FIG. 37 is a flow chart illustrating the operations for friction stirwelding, according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

Referring now to the drawings and, in particular, to FIG. 2B, there isshown a friction stir welding device or apparatus 20, according to oneembodiment of the present invention, for friction stir welding aworkpiece or workpieces 23, such as the workpieces 23 shown in FIG. 2A.The friction stir welding device 20 of the present invention isparticularly suited for friction stir welding large workpieces andworkpieces having curvilinear geometries. The friction stir weldingdevice 20 includes a friction stir welding tool 22 and a device ormachine (not shown) structured for rotating a friction stir weldingtool, such as a milling machine or drill having a spindle 21 thatpreferably includes independently rotatable and axially translatableouter and inner portions 21 a, 21 b. The outer and inner portions 21 a,21 b of the spindle 21 preferably can be translated axially independentof one another. The device or machine structured for rotating thefriction stir welding tool 22 can be operated manually, but preferablyis operated by a computer, microprocessor, microcontroller or the likeoperating under software control. According to one embodiment, thedevice or machine structured for rotating the friction stir welding tool22 includes a 55UA bobbin tool weld machine manufactured by ESAB.

As illustrated in FIG. 3, the friction stir welding tool 22 includes afirst shoulder 24, a second shoulder 25, and a pin 26 extendingtherebetween. Referring to FIGS. 27-34, the first shoulder 24 can beformed in a variety of configurations, but preferably is cylindrical inshape. The first shoulder 24 has a body 29 defining an aperture 32extending the length of the first shoulder. The body 29 of the firstshoulder 24 preferably is formed of a material having high strength andheat resistance. For purposes of example only and not limitation, thebody 29 of the first shoulder 24 can be constructed of tool steel, amolybdenum alloy, such as TZM, and nickel alloys, such as Rene 41 (UNSN07041). The first end 29 a of the body 29 of the first shoulder 24 isstructured to be in rotatable communication with the outer rotatableportion 21 a of the spindle 21 so that the first shoulder is inrotatable communication with the spindle. For example, the outerrotatable portion 21 a of the spindle 21 can include a conventionalchuck or collet (not shown) that engages the exterior of the body 29 ofthe first shoulder 24. In another embodiment (not shown), the aperture32 of the first shoulder 24 defines a threaded portion at the first end29 a that is structured to threadably engage a corresponding threadedportion extending from, or defined by, the outer rotatable portion 21 aof the spindle 21. In yet another embodiment (not shown), the exteriorof the body 29 of the first shoulder 24 defines a threaded portion atthe first end 29 a that is structured to threadably engage acorresponding threaded portion defined by the outer rotatable portion 21a of the spindle 21.

As illustrated in FIGS. 28 and 32, the diameter of the aperture 32 ofthe first shoulder 32 is larger at the first end 29 a of the body 29than at the second end 29 b of the body. More specifically, the diameterof the aperture 32 gradually tapers from the first end 29 a to thesecond end 29 b of the first shoulder 24 until just before the secondend 29 b of the first shoulder where the aperture 32 exhibits arelatively sharp decrease in diameter. The sharp decrease in diameter ofthe aperture 32 of the first shoulder 24 defines an angled interiorsurface 34 proximate to and surrounding the aperture 32 at the secondend 29 b of the first shoulder.

As illustrated in FIGS. 29, 30, 33 and 34, the second end 29 b of thefirst shoulder 24, which is adjacent the workpiece 23 in FIG. 2B, has asurface 35 structured to frictionally engage the workpiece to thereby atleast partially form the friction stir weld joint 19. For example, inone embodiment, as illustrated in FIGS. 29 and 33, the surface 35 isthreaded 36. In another embodiment (not shown), the surface 35 has aconvex configuration. In still another embodiment, the surface 35 has aconcave configuration, similar to the shoulder 16 illustrated in FIG.1B, to capture and consolidate the plasticized material in the weldjoint 19. In yet another embodiment, the surface 35 defines one or moreraised portions or surfaces, such as threads 36, bumps, or ribs,structured to frictionally engage the workpiece 23. In yet anotherembodiment, the surface 35 has a relatively flat or planarconfiguration.

Referring to FIGS. 4-17, the pin 26 has first and second ends 30, 31 anda stirring portion 37 positioned along the length of the pin between thefirst and second ends. The pin 26 preferably is formed of a materialhaving high strength and heat resistance. For purposes of example onlyand not limitation, the pin 26 can be constructed of tool steel, amolybdenum alloy, such as TZM, and nickel alloys, such as Rene 41 (UNSN07041). As illustrated in FIGS. 4 and 5, the first end 30 of the pin 26preferably defines a threaded portion 28, which as illustrated in FIG.2B, is structured to threadably engage a corresponding threaded aperture(not shown) within the adapter or shaft 27 that extends from the pin 28to the inner rotatable portion 21 b of the spindle 21 so that the pin isin rotatable communication with the spindle. In another embodiment (notshown), the first end 30 of the pin 26 is of sufficient length toconnect directly to the inner rotatable portion 21 b of the spindle 21using a threaded connection or a conventional chuck or collet (notshown).

Referring to FIGS. 4 and 5, the first end 30 of the pin 26 defines aprotuberance 40 adjacent the threaded portion 28 of the pin. ComparingFIG. 4 to FIGS. 28 and 32, the protuberance 40 includes an angledexterior surface 40 a having a relatively sharp decrease in diameterwhich corresponds generally to the angled interior surface 34 defined bythe aperture 32 of the first shoulder 24 at the second end 29 b of thebody 29. The dimensions of the pin 26 and protuberance 40 are such thatthe second end 31 of the pin and the protuberance will slide through theaperture 32 of the first shoulder 24 from the first end 29 a to thesecond end 29 b until the exterior surface 40 a of the protuberancerests against the angled interior surface 34 defined by the aperture 32.In this position, the second end 31 of the pin 26 extends from thesecond end 29 b of the first shoulder 24 through the aperture 32 definedtherein. The pin 26 and protuberance 40 preferably are free to rotatewithin the aperture 32 of the first shoulder 24. The angled interiorsurface 34 of the first shoulder 24 preferably is machined to a finefinish, which, in combination with the thickness of the body 29 of thefirst shoulder, acts as a bearing between the contact surfaces of thepin 26 and the first shoulder 24 to reduce friction therebetween. Thethickness of the body 29 of the first shoulder 24 preferably is about0.375 inches or more.

Referring to FIGS. 4, 6, 7, 8, 9, 10, 11, 12, 14, 15, and 17, the pin 26has a stirring portion 37 that can be configured in a variety ofconfigurations depending on the dimensions and material properties ofthe workpiece 14. For example, in one embodiment, as illustrated inFIGS. 6, 7, 10 and 11, the stirring portion 37 of the pin 26 defines oneor more planar surfaces 52. In another embodiment, as illustrated inFIGS. 6, 7, 10 and 11, the stirring portion 37 of the pin 26 defines atleast one threaded surface 54 and at least one planar surface 52. Inanother embodiment, as illustrated in FIG. 4, the stirring portion 37 ofthe pin 26 comprises a first threaded surface 54 a having threadsoriented in a first direction and a second threaded surface 54 b havingthreads oriented in a second direction, and wherein the first directionis different from the second direction. In yet another embodiment, asillustrated in FIGS. 8, 9, 12, and 15, the stirring portion 37 of thepin 26 comprises a plurality of concave surfaces 55.

Referring to FIGS. 18-21 and 23-26, the second shoulder 25 can be formedin a variety of configurations, but preferably is cylindrical in shape.The second shoulder 25 has a body 39 defining an aperture 42 at least atthe first end 39 a thereof. The body 39 of the second shoulder 25preferably is formed of a material having high strength and heatresistance, and high thermal conductivity. For purposes of example onlyand not limitation, the body 39 of the second shoulder 25 can beconstructed of tool steel, a molybdenum alloy, such as TZM, and nickelalloys, such as Rene 41 (UNS N07041). The first end 39 a of the body 39of the second shoulder 25 is structured to be in rotatable communicationwith second end 31 of the pin 26 and, thus, the inner rotatable portion21 b of the spindle 21. For example, as illustrated in FIGS. 21 and 22,the second end 31 of the pin 26 can be threaded 33 and the aperture 42of the body 39 of the second shoulder 25 can be threaded 46 such thatthe second end of the pin is threadably received within the aperture 42.Alternatively, as illustrated in FIG. 26, the second end 31 of the pin26 can define a polygonal surface (not shown) and the aperture 42 of thebody 39 of the second shoulder 25 can define a corresponding polygonalconfiguration 43 so that the second end 31 of the pin 26 is matinglyreceived within the aperture 42. A mechanical fastener (not shown), suchas a cotter pin or setscrew, can be used to secure the second end 31 ofthe pin 26 within the aperture 42 of the second shoulder 25. Asillustrated in FIGS. 21, 22, and 26, the aperture 42 defined by the body39 of the second shoulder 25 can be configured to receive the tip 31 aof the second end 31 of the pin 26. As illustrated in FIGS. 21 and 26,the aperture 42 can extend through the entire length of the body 39 ofthe second shoulder 25 from the first end 39 a to the second end 39 b oronly a portion thereof.

As illustrated in FIGS. 18, 21, 23, and 26, the body 39 of the secondshoulder 25 preferably defines one or more fins 50 structured totransfer heat away from the weld joint 19. The fins 50 can beparticularly advantageous when friction stir welding workpieces 23formed of different materials in which one workpiece has a lower solidustemperature than the other. The fins 50 transfer heat away from theworkpiece adjacent the second shoulder 25, which has the lower solidustemperature, thereby lowering the temperature of the workpiece below thetemperature of the workpiece adjacent the first shoulder 24, which hasthe higher solidus temperature. In another embodiment (not shown), thefins 50 are formed on the first shoulder 24, instead of the secondshoulder 26, and the workpiece having the lower solidus temperature ispositioned adjacent the first shoulder.

As illustrated in FIGS. 19, 21, 24 and 26, the first end 39 a of thebody 39 of the second shoulder 25, which is adjacent the workpiece 23 inFIG. 2B, has a surface 45 structured to frictionally engage theworkpiece to thereby at least partially form the friction stir weldjoint 19. For example, in one embodiment, as illustrated in FIGS. 19 and24, the surface 45 is threaded 48. In another embodiment (not shown),the surface 45 has a convex configuration. In still another embodiment,the surface 45 has a concave configuration, similar to the shoulder 16illustrated in FIG. 1B, to capture and consolidate the plasticizedmaterial in the weld joint 19. In yet another embodiment, the surface 45defines one or more raised portions or surfaces, such as threads 48,bumps, or ribs, structured to frictionally engage the workpiece 23. Inyet another embodiment, the surface 45 has a relatively flat or planarconfiguration.

Prior to operation of the friction stir welding device 20, the frictionstir welding tool 22 is secured to the spindle 21. According to oneembodiment, the first end 30 of the pin 26 is first attached to theinner portion 21 b of the spindle 21. For example, the first end 30 ofthe pin 26 and the inner portion 21 b of the spindle 21 can includemating threads, or the inner portion 21 b of the spindle can include aconventional chuck or collet. The second end 31 of the pin 26 is theninserted through the aperture 32 defined by the first end 29 a of thebody 29 of the first shoulder 24 so that the second end 31 of the pinextends through the aperture 32 at the second end 29 b of the body 29 ofthe first shoulder. The first shoulder 24 is attached to the outerportion 21 a of the spindle 21. For example, the first shoulder 24 andthe outer portion 21 a of the spindle 21 can include mating threads, orthe outer portion 21 a of the spindle can include a conventional chuckor collet. The second end 32 of the pin 26 is then inserted into, andsecured within, the aperture 42 defined by the second shoulder 25. Forexample, the second end 32 of the pin 26 and the aperture 42 of thesecond shoulder 25 can include mating threads, or the second end 32 ofthe pin and the aperture 42 of the second shoulder can definecorresponding male and female polygonal configurations. A mechanicalfastener (not shown), such as a conventional cotter pin or setscrew, canbe inserted into the second shoulder 25 to secure the second end 32 ofthe pin 26 within the aperture 42. In another embodiment, an aperture(not shown) structured to receive the second end 32 of the pin 26 can bepre-machined into the workpiece or workpieces 23, in which case thesecond end 32 of the pin is inserted through the aperture and thenattached to the second shoulder 25.

Once the friction stir welding tool 22 is attached to the spindle 21, afriction stir weld joint 19 is formed. According to one embodiment ofthe present invention, the outer portion 21 a of the spindle 21 isrotated to thereby rotate the first shoulder 24 and the inner portion 21b of the spindle is rotated to thereby rotate the pin 26 and the secondshoulder 26. The spindle 21 preferably is structured such that the speedand direction of rotation, i.e., the “angular velocity”, of the outerportion 21 a and the inner portion 21 b can be varied independently ofone another. For example, the outer portion 21 a of the spindle 21 canrotate the first shoulder 24 in the same direction and speed of rotationas the inner portion 21 b of the spindle 21 rotates the pin 26 andsecond shoulder 25. Alternatively, the outer portion 21 a of the spindle21 can rotate the first shoulder 24 at a different speed and/ordirection than the inner portion 21 b of the spindle 21 rotates the pin26 and second shoulder 25.

In order to form a friction stir weld joint 19, the rotating frictionstir welding tool 22 is moved into contact with the workpiece orworkpieces 23 so that the rotating surfaces 35, 45 of the first andsecond shoulders 24, 25, respectfully, and the stirring portion 37 ofthe pin 26 frictionally engage the workpiece or workpieces. The rotatingfriction stir welding tool 22 preferably is moved through the workpieceor workpieces along a predetermined path to thereby form an elongateweld joint. During friction stir welding, the portions of the workpieceor workpieces 23 proximate to the pin 26 are “sandwiched” between thefirst shoulder 24 and the second shoulder 25. Advantageously, the forceexerted by the surfaces 35, 45 of the first and second shoulders 24, 25compresses the workpiece or workpieces 23 about the pin 26 creating aseal that prevents the plasticized material from being extruded.

According to one embodiment of the present invention (not shown), thedevice or machine structured for rotating the friction stir welding tool22 can be structured to rotate and axially translate the first shoulder24 or the combined pin 26 and second shoulder 25, or both, relative toone another in response to changes in the magnitude of the force exertedby the first shoulder and/or second shoulder on the workpiece(s) 23 andthe weld joint 19. For example, the friction stir weld device 20 caninclude means for measuring the magnitude of the force exerted by thefirst and/or second shoulder 24, 25 upon the workpiece(s) 23 and theweld joint 19. The means for measuring the magnitude of the force caninclude a computer, a microprocessor, a microcontroller or the likeoperating under software control, which is in electrical communicationwith at least one sensor. The sensor can include a strain-gage loadcell, a piezoelectric load cell, a dynamometer, a pneumatic load cell,or a hydraulic load cell that is positioned on a workpiece 23, the firstshoulder 24, and/or the second shoulder 25.

During operation, measurements from the at least one sensor areperiodically transmitted or communicated to the computer,microprocessor, or microcontroller through suitable electrical oroptical wiring. The computer, microprocessor, or microcontroller thencompares the magnitude of the force measured by the sensor to apredetermined value. If the magnitude of the force communicated from thesensor differs from the predetermined value, then the computer,microprocessor, or microcontroller transmits a signal to the device ormachine structured for rotating the friction stir welding tool 22instructing the device or machine to modify the force exerted by thefirst shoulder 24 and the second shoulder 25 on the workpiece(s) 23 andthe weld joint 19. This process can be repeated based on subsequentmeasurements transmitted by the sensor to the computer, microprocessor,or microcontroller until the magnitude of the force is equal to, orapproximates, the predetermined value.

As discussed above, the device or machine structured for rotating thefriction stir welding tool 22 can include a 55UA bobbin tool weldmachine manufactured by ESAB, which includes a spindle 21 having anouter portion 21 a, which is structured for rotating and axiallytranslating the first shoulder 24, and an inner portion 21 b, which isstructured for rotating and axially translating the pin 26 and thesecond shoulder 25. The device or machine can be operated manually orautomatically by connecting the device or machine, using suitableelectrical or optical wiring (not shown), to a computer, microprocessor,microcontroller or the like operating under software control.

If the magnitude of the force communicated from the sensor is less thanthe predetermined value, then the computer, microprocessor, ormicrocontroller transmits a signal to the device or machine structuredfor rotating the friction stir welding tool 22 instructing the device ormachine to increase the force exerted by the first and second shoulders24, 25 on the workpiece(s) 23 and the weld joint 19. For example, theforce on the workpiece(s) 23 and the weld joint 19 can be increased byurging the pin 26 and the second shoulder 25 toward the first shoulder24 by axially translating the inner portion 21 b of the spindle 21 in adirection away from the first end 29 a of the body 29 of the firstshoulder. Alternatively or simultaneously, the force on the workpiece(s)23 can be increased by urging the first shoulder 24 toward the secondshoulder 25 by axially translating the outer portion 21 a of the spindle21 in a direction toward the second shoulder.

If the magnitude of the force communicated from the sensor is more thanthe predetermined value, then the computer, microprocessor, ormicrocontroller transmits a signal to the device or machine structuredfor rotating the friction stir welding tool 22 instructing the device ormachine to decrease the force exerted by the first and second shoulders24, 25 on the workpiece(s) 23 and the weld joint 19. For example, theforce on the workpiece(s) 23 and the weld joint 19 can be decreased byurging the pin 26 and the second shoulder 25 away from the firstshoulder 24 by axially translating the inner portion 21 b of the spindle21 in a direction toward the first end 29 a of the body 29 of the firstshoulder. Alternatively or simultaneously, the force on the workpiece(s)23 and the weld joint 19 can be decreased by urging the first shoulder24 away from the second shoulder 25 by axially translating the outerportion 21 a of the spindle 21 in a direction away from the secondshoulder.

In another embodiment of the present invention (not shown), the frictionstir weld device 20 includes means responsive to the measuring means,for axially translating the pin 26 relative to the second end 29 b ofthe first shoulder 24 so as to axially translate the second shoulder 25toward or away from the workpiece(s) 23 in order to modify the forceexerted by the first and second shoulders 24, 25 upon the workpiece(s)and the weld joint 19. The means responsive to the measuring means caninclude cams and a follower, a power screw, or an actuator assembly,such as one or more pneumatic or hydraulic arms. For example, theassignee of the present application has developed methods and apparatusfor controlling the position of a friction stir welding probe, asdisclosed in commonly owned U.S. patent application Ser. No. 09/087,416entitled “Method and Apparatus for Controlling Downforce During FrictionStir Welding” filed on May 29, 1998, the entire disclosure of which ishereby incorporated by reference.

The workpieces 23 joined or welded, according to the present invention,can be formed of either similar or dissimilar metals. Advantageously,since the workpieces 23 are joined by friction stir welding, theworkpieces 23 can be formed of dissimilar metals that would beunweldable or uneconomical to join by conventional fusion weldingtechniques. Unweldable materials, when joined by conventional fusionwelding techniques, produce relatively weak weld joints that tend tocrack during weld solidification. Such materials include aluminum andsome aluminum alloys, particularly AA series 2000 and 7000 alloys. Theuse of friction stir welding permits workpieces 23 formed of unweldablematerials to be securely joined. Friction stir welding also can be usedto securely join weldable materials to other weldable and to unweldablematerials. For example, one or both of the workpieces 23 can be formedof aluminum, aluminum alloys, titanium, or titanium alloys. Thus, thematerials that form the workpieces 23 can be chosen from a wider varietyof light weight, high strength metals and alloys, thereby facilitatingreduction of the overall weight of the resulting structural assembly,which is a critical concern in the aerospace industry.

Referring to FIG. 35, there is illustrated a lap joint 56, according toone embodiment of the present invention, formed using the friction stirwelding tool 22. Although the friction stir welding tool 22 can be usedto form a variety of different types of weld joints, including, but notlimited to, butt joints, edge joints, and lap joints, the presentinvention is particularly useful in forming lap joints and edge jointsformed of two or more workpieces or structural members 23 a, 23 b thathave different solidus temperatures. As illustrated in FIG. 35, thesecond structural member 23 b at least partially overlaps the firststructural member 23 a so as to define an interface 57 therebetween. Thelap joint 56 includes a friction stir weld joint 58 joining the firstand second structural members 23 a, 23 b at least partially at theinterface 57. The friction stir weld joint 58 defines first and secondportions 58 a, 58 b. The first portion 58 a of the friction stir weldjoint 58 is mixed by the friction stir weld tool 22 and, morespecifically, the surface 45 of the second shoulder 25, at a firstangular velocity, and the second portion 58 b of the friction stir weldjoint 58 is mixed by the friction stir welding tool 22 and, morespecifically, the surface 35 of the first shoulder 24, at a secondangular velocity to thereby form grain structures of differentrefinement in the first and second portions 58 a, 58 b. As used herein,“angular velocity” includes both a speed component and a directioncomponent. The direction component is positive for motion following the“right hand rule,” i.e., counter-clockwise motion, and is negative formotion in the opposite direction, i.e., clockwise motion.

Advantageously, because the surfaces 35, 45 of the first and secondshoulders 24, 25 can be rotated at different angular velocities whenforming the weld joint 19, the materials used to construct the first andsecond structural members 23 a, 23 b can comprise not only dissimilarmetals, but materials that have different solidus temperatures, as wellas different hardnesses, thus further expanding the range of materialsthat can be used to construct the resulting structural assembly.

Referring to FIG. 36, there are illustrated the operations performed forfriction stir welding a workpiece or workpieces 23 using the frictionstir welding device 20, according to one embodiment of the presentinvention. The method includes positioning first and second shouldersadjacent the workpiece. See Block 60. Each of the first and secondshoulders has a surface structured to frictionally engage the workpiece.A pin is connected to the first and second shoulders so that the pinextends therebetween. See Block 61. The pin defines a stirring portionstructured to frictionally engage the workpiece. Thereafter, the firstshoulder is rotated at a first angular velocity and the pin and thesecond shoulder are rotated at a second angular velocity different fromthe first angular velocity so that at least a portion of each of thepin, the first shoulder, and the second shoulder frictionally engagesthe workpiece to thereby form a friction stir weld joint. See Block 68.The stirring portion of the pin can be moved through the workpiece alonga predetermined path. See Block 72.

Referring to FIG. 37, there are illustrated the operations performed forfriction stir welding a workpiece or workpieces 23 using the frictionstir welding device 20, according to another embodiment of the presentinvention. The method includes positioning first and second shouldersadjacent the workpiece. See Block 80. Each of the first and secondshoulders has a surface structured to frictionally engage the workpiece.A pin is connected to the first and second shoulders so that the pinextends therebetween. See Block 81. The pin defines a stirring portionstructured to frictionally engage the workpiece. The first shoulder isrotated. See Block 88. Concurrently with the first rotating step, thepin and the second shoulder are rotated independently of the firstshoulder so that at least a portion of each of the pin, the firstshoulder, and the second shoulder frictionally engages the workpiece tothereby form a friction stir weld joint. See Block 89. For example, thefirst and second rotating steps can include rotating the first andsecond shoulders at the same angular velocity. In another embodiment,the first and second rotating steps include rotating the first shouldera first angular velocity and the pin and the second shoulder at a secondangular velocity, wherein the second angular velocity is different fromthe first angular velocity. See Block 90. As used herein, “angularvelocity” includes both a speed component and a direction component. Thedirection component is positive for motion following the “right handrule,” i.e., counter-clockwise motion and is negative for motion in theopposite direction, i.e., clockwise motion. The stirring portion of thepin can be moved through the workpiece along a predetermined path. SeeBlock 94.

As illustrated in FIGS. 36 and 37, the method of connecting the pin tothe first and second shoulders can be varied. In one embodiment, theconnecting step comprises sliding an end of the pin through an aperturein the first shoulder. See Blocks 62 and 82. The connecting step canthen include threading an end of the pin into a threaded aperturedefined by the second shoulder. See Blocks 63 and 83. In anotherembodiment, the connecting step comprises drilling an aperture in theworkpiece. See Blocks 64 and 84. An end of the pin is slid through anaperture in the first shoulder. See Blocks 65 and 85. The end of the pinis inserted through the aperture in the workpiece and connected to thesecond shoulder. See Blocks 66 and 86. The end of the pin is thenconnected to the second shoulder. See Blocks 67 and 87.

As illustrated in FIGS. 36 and 37, the position of the first and secondshoulders relative to one another can be modified in order to adjust theforce exerted by the shoulders on the workpiece. See Blocks 69 and 91.For example, in one embodiment, the pin is urged toward the firstshoulder so as to urge the second shoulder toward the first shoulder.See Blocks 70 and 92. In another embodiment, the first shoulder is urgedtoward the second shoulder. See Blocks 71 and 93.

Accordingly, there has been provided a friction stir welding tool,apparatus and associated method of manufacture for forming weld jointsby friction stir welding large workpieces or workpieces havingcurvilinear geometries. The tool is capable of effectively supporting aweld joint and constraining the plasticized material within the weldjoint during friction stir welding. The tool can easily be adapted tovarying workpiece geometries and sizes. In addition, the tool allows forfriction stir welding workpieces having different material properties.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

1. A method of friction stir welding a workpiece, comprising:positioning first and second shoulders adjacent the workpiece, each ofthe first and second shoulders having a surface structured tofrictionally engage the workpiece; connecting a pin to the first andsecond shoulders so that the pin extends therebetween, the pin defininga stirring portion structured to frictionally engage the workpiece; andthereafter, rotating the first shoulder at a first angular velocity andthe pin and the second shoulder at a second angular velocity differentfrom the first angular velocity so that at least a portion of each ofthe pin, first shoulder, and second shoulder frictionally engages theworkpiece to thereby form a friction stir weld joint.
 2. A methodaccording to claim 1 wherein said connecting step comprises sliding anend of the pin through an aperture in the first shoulder.
 3. A methodaccording to claim 1 wherein said connecting step comprises threading anend of the pin into a threaded aperture defined by the second shoulder.4. A method according to claim 1 wherein said connecting step comprises:drilling an aperture in the workpiece; sliding an end of the pin throughan aperture in the first shoulder; inserting the end of the pin throughthe aperture in the workpiece; and connecting the end of the pin to thesecond shoulder.
 5. A method according to claim 1 further comprisingmoving the stirring portion of the pin through the workpiece along apredetermined path.
 6. A method according to claim 1 further comprisingurging the pin toward the first shoulder so as to urge the secondshoulder toward the first shoulder.
 7. A method of friction stir weldinga workpiece, comprising: positioning first and second shoulders adjacentthe workpiece, each of the first and second shoulders having a surfacestructured to frictionally engage the workpiece; connecting a pin to thefirst and second shoulders so that the pin extends therebetween, the pindefining a stirring portion structured to frictionally engage theworkpiece; rotating the first shoulder; and concurrently with said firstrotating step, rotating the pin and the second shoulder independently ofthe first shoulder so that at least a portion of each of the pin, firstshoulder, and second shoulder frictionally engages the workpiece tothereby form a friction stir weld joint.
 8. A method according to claim7 wherein said connecting step comprises sliding an end of the pinthrough an aperture in the first shoulder.
 9. A method according toclaim 7 wherein said connecting step comprises threading an end of thepin into a threaded aperture defined by the second shoulder.
 10. Amethod according to claim 7 wherein said connecting step comprises:drilling an aperture in the workpiece; sliding an end of the pin throughan aperture in the first shoulder; inserting the end of the pin throughthe aperture in the workpiece; and connecting the end of the pin to thesecond shoulder.
 11. A method according to claim 7 further comprisingmoving the stirring portion of the pin through the workpiece along apredetermined path.
 12. A method according to claim 7 further comprisingurging the pin toward the first shoulder so as to urge the secondshoulder toward the first shoulder.
 13. A method according to claim 12wherein said first and second rotating steps comprise rotating the firstshoulder at a first angular velocity and the pin and the second shoulderat a second angular velocity, the second angular velocity beingdifferent from the first angular velocity.